This invention relates to current sources and more specifically to current sources having a process selectable temperature coefficient.
FIG. 1 illustrates the current versus voltage (IV) characteristic curve for an ideal current source. As is known in the art, it is impossible to build an ideal current source and therefore the best that can be obtained is an approximation of the curve of FIG. 1. FIG. 2 illustrates the IV characteristic curve of a typical current source manufactured using conventional components. As can be seen, the curve of FIG. 2 includes a first region 1 in which current first increases from zero amperes to a desired current I.sub.s, a second region 2 in which the current only varies minimally as a function of voltage, and a region 3 in which current increases in response to an excessive voltage applied across the current source.
A typical current source which provides an IV characteristic such as the curve of FIG. 2 is a depletion mode MOS field effect transistor (MOSFET) Q1 illustrated in FIG. 3a. MOSFET Q1 is an N-channel transistor. A current source using a depletion mode P-channel transistor is illustrated in FIG. 3b. As is known in the art, transistors Q1 and Q2 will each conduct a constant current and thus serve as a current source when a voltage exceeding the transistor pinch-off voltage is applied across their source and drain.
It is also known in the art to manufacture current sources using DMOS transistors. (A DMOS transistor is a transistor in which the transistor channel length is defined by the difference in diffusion of sequentially introduced impurities from a common edge or boundary. DMOS transistors are described. for example, in "Power MOS Devices in Discrete and Integrated Circuits" by Plummer et al., published by Stanford University in 1984 and incorporated by reference.) Such current sources operate in a manner similar to MOSFET current sources.
Because typical current sources are designed using MOSFETs or DMOS FETs, the output current of such current sources is also typically dependent upon temperature. In many applications, however, it is desirable to have a current source which is either temperature independent or has a selected temperature dependence.